The present invention relates to a method and apparatus for coating a substrate such as a semiconductor wafer or an LCD (liquid crystal display) substrate with resist, followed by developing the resist coating.
Used in a photolithography process is a resist processing system in which semiconductor wafers are successively coated with a resist solution and, then, the resist coating is developed. Many process units are included in a process section of the resist processing system. For example, in the resist processing system disclosed in U.S. patent application Ser. No. 08/667,712, U.S. Pat. No. 5,700,127 many process units are arranged on both sides of a horizontal transfer path, and a wafer is transferred by a main arm mechanism along the horizontal transfer path into each process unit for a desired processing. Also, in the resist processing system disclosed in U.S. Pat. No. 5,664,254, many process units are arranged around a vertical transfer path, and a wafer is transferred by a main arm mechanism along the vertical transfer path into each process unit for a desired processing.
For processing a wafer accurately and with safety within each process unit, the wafer must be positioned with high accuracy within the process unit. It was customary in the prior art to introduce a wafer, which is transferred from a cassette section into a process section, into an alignment unit for adjusting the wafer position relative to the process unit within the process section. Also, every time the position of a wafer taken out of a process unit is deviated from the position in the step of transfer into the process unit, the wafer is transferred into the alignment unit to adjust the wafer position as desired relative to the next process unit. Particularly, in the resist coating or developing unit in which the wafer is revolved about its own axis during the processing, the horizontal position of the orientation flat of the wafer taken out of the process unit differs from that in the step of transfer into the process unit. Therefore, the wafer must be transferred to the alignment unit for adjusting the horizontal position of the orientation flat every time the wafer is taken out of the particular process unit. In this fashion, the main arm mechanism must be moved for simply adjusting the wafer position. It follows that a load given to the main arm mechanism is increased. In addition, the wafer transfer time is increased, leading to a low through-put and an increased danger of particle adhesion to the wafer.
In the resist coating unit, the inner surface of a drain cup is periodically washed using an exclusive washing tool containing a solvent such as a thinner in order to remove the resist attached to the inner surface of the cup. In washing the cup, the washing tool housed somewhere in the system is transferred to the resist coating unit so as to be mounted to a spin chuck. When the washing tool mounted to the spin chuck is rotated, the solvent is spurted through holes made in the circumferential wall of the tool toward the inner surface of the drain cup so as to dissolve the resist attached to the inner surface of the cup. After the washing, the washing tool is detached from the spin chuck and is brought back to the original housing region. It should be noted that, if the washing tool fails to be brought back accurately to the original position, the washing tool cannot be mounted to the spin chuck when the drain cup is washed again. To overcome this difficulty, the washing tool is transferred to the alignment unit after the washing operation for positioning the washing tool as desired. Then, the washing tool is brought back to the original housing region. The requirement of transferring the washing tool to the alignment unit naturally causes the through-put to be lowered.